The use and limitations of chiasma scoring with reference to human genetic mapping

Human chiasma data are summarized, and some preliminary new observations in fetal oocytes are presented. Male chiasma data may give reliable estimates of genetic lengths, both for individual chromosome arms and for the total autosomal complement. Female data are as yet less accurate and give information according to chromosome group only. Movement of chiasmata before they can be reliably scored is unlikely. In both sexes, chiasmata are seen to be clustered along the length of the chromosomes, which may reflect crossingover interference and a tendency for crossingover to more often take place in certain chromosome segments; there are some indications of sex differences in these preferences.